The present invention relates to a field-effect transistor (hereafter referred to as the FET) using a compound semiconductor. More particularly, the present invention concerns a so-called high electron mobility transistor of a pseudomorphic type in which an indium-gallium-arsenide (hereafter referred to as InGaAs) semiconductor crystal layer containing indium (hereafter referred to as In) and an aluminum-gallium-arsenide (hereafter referred to as AlGaAs) layer are formed on a gallium-arsenide (hereafter referred to as GaAs) semiconductor substrate, thereby constituting an FET.
In recent years, the use of semiconductors in high-frequency regions has become active, FETs operating at high speed at high frequencies have been required, and high electron mobility transistors using compound semiconductors have been developed. To effect higher speed, the so-called pseudomorphic-type FETs in which InGaAs whose electron mobility is greater than that of GaAs is used in a channel layer have been developed.
The basic structure of this pseudomorphic-type FET is such that In.sub.0.15 Ga.sub.0.85 As layer whose In composition ratio is, for example, 15% is formed on a GaAs substrate, AlGaAs is formed thereon, and a drain electrode, a source electrode, and a gate electrode are formed on that surface.
In the FET of this structure, the channel layer through which electrons travel is formed in the InGaAs layer, its thickness is small in the vicinity of about 150 .ANG., and the substrate side thereof abuts against GaAs having small electron affinity, so that electrons are sealed in a quantum well in InGaAs. As a result, this type of FET has an outstanding characteristic that, as compared with a conventional GaAs-type FET, g.sub.m is larger, and the short channel effect is smaller.
Energy band diagrams in cases where 0 V and negative voltage are applied to the gate electrode are respectively shown in FIGS. 5 and 6. In these drawings, the abscissa represents the depth from the gate electrode side in the lower direction of the GaAs substrate, point A indicates an interface between the gate electrode and the AlGaAs layer, point B indicates an interface between the AlGaAs layer and the InGaAs layer, and point C indicates an interface between the InGaAs layer and the GaAs substrate, while the ordinate represents energy. In addition, E shows a distribution of electrons in the direction of the depth of the semiconductor substrate when the FET is operated. It can be appreciated from FIGS. 5 and 6 that the distribution of electrons upon application of the negative voltage to the gate electrode becomes wider in the direction of the depth of the substrate, and a distance a from the surface (point A) of the substrate to the center (point D) of the distribution of electrons is large.
Since the InGaAs layer containing In is formed in the channel layer as described above, the ability is improved. However, there still exists the problem that when the negative voltage is applied to the gate electrode, the mutual conductance g.sub.m of the transistor becomes small. Also, there is a problem in that the noise figure becomes aggravated in a region where the drain current is small.
That is, g.sub.m is expressed by the following Formula (1): ##EQU1## where .mu. is the mobility of electrons, Lg is a gate length, Wg is a gate width, e is a dielectric constant of AlGaAs, Vg is a gate voltage, Vth is a threshold voltage, and a is a distance (see FIG. 5 or 6) between the gate electrode and the channel. If the negative voltage is applied to the gate electrode, the distribution of electrons becomes farther from the substrate surface, and a becomes large. As a result, g.sub.m becomes small from Formula (1).
In addition, the noise figure NF is expressed by the following Formula (2): ##EQU2## where K.sub.f is a fitting factor which is determined by the material, configuration, and the like of the semiconductor, f is a frequency during operation, Cgs is a gate capacitance, g.sub.m is mutual conductance, Rs is source resistance, and Rg is gate resistance. It can be appreciated from this Formula (2) that if g.sub.m becomes small, NF becomes large, and the noise figure becomes large.